1. Field of the Invention
The invention relates to a method of producing platelets having a thickness of 0.2-1000 xcexcm and a diameter of 0.5 -10,000 xcexcm.
2. Background Art
A variety of platelets occur in nature as flakes, nacre or mica platelets and are used either as such or in surface-modified form as special-effect pigments. In addition, synthetic special-effect pigments are known which, for example, comprise thin platelets of cholesteric liquid crystals. Furthermore, such platelets are widely used in other applications, for example as rheological additives.
WO 93/08237, DE 196 18 563 A1, DE 196 18 566 A1 and DE 197 07 805 A1 describes the preparation of interference pigments which consist of high and low refractive index, optionally colored or fully absorbing laminae and of metal laminae. These interference pigments are prepared by one or a plurality of interference pigment materials being applied to a substrate material, the thusly formed layer then being stripped from the substrate. This layer must not, even when composed of individual sublayers, separate into its component sublayers during the stripping process and subsequent processing steps, since this would cause the characteristic properties of the interference pigment to be lost. A sublayer is therefore an integral component of a platelet produced according to the prior art.
If the materials for preparing the layer have low mechanical strength, the material layer cannot be pulled off the substrate material over its entire area without tearing, but instead has to be subjected to an abrasive process. Particularly with thin layers below 10 xcexcm, this process step is cost-intensive and entails wear problems relating to the substrate material, particularly if a terephthalate sheet (DE 196 18 566 A1, DE 197 07 805 A1), commonly used as a substrate material, is involved. To avoid this problem it is known to apply release agents to the substrate material, before the materials forming the interference pigment are applied. This procedure is eminently suitable if the interference pigment is formed by brittle materials. Considerable problems arise, however, if materials having high flexural elongations and low ultimate tensile strength are involved. On the one hand, it is then extremely difficult to convert these layered materials into fragments which can be removed from the substrate by means of compressed air or a suction operation, and on the other hand they do not have sufficient mechanical strength to be stripped undamaged as a film from the substrate.
U.S. Pat. No. 5,364,557 discloses the preparation of thin cholesteric platelets by applying a cholesteric layer to a substrate material, e.g. onto a sheet by means of an applicator mechanism consisting of a doctor blade or a roll applicator mechanism. In the case of fusible materials, application takes place above the melting temperature or glass transition temperature of the material. Then the material is removed from the substrate at a temperature below the melting temperature or glass transition temperature by means of an air jet which optionally includes an abrasive powder, or alternatively, by means of a scraper. A special embodiment described is the transfer to a further substrate material. Also disclosed is the generation of layers consisting of substances having different properties. Described, inter alia, are bilayers consisting of left- and right-helical material which, in the course of size reduction process, afford platelets which consist both of a left- and of a right-helical lamina and do not disintegrate into further laminae during the size reduction process.
All the procedures disclosed in U.S. Pat. No. 5,364,557 lead to a single lamina, which may be composed of sublaminae and which, upon crushing, afford platelets which are of the same thickness as the layer subjected to the grinding process.
DE 42 40 743, corresponding to U.S. Pat. No. 5,362,315, likewise discloses the preparation of crosslinked cholesteric platelets via single layers.
According to EP 07 93 693, the production of crosslinked cholesteric platelets is effected by printing cholesteric material onto a substrate, by crosslinking and by subsequent removal of the printed, crosslinked material. This process, too, is based on a single layer which corresponds to the cholesteric platelet ultimately obtained.
The usefulness of all the abovementioned methods is dependent on how easy it is to strip the film which is either crosslinked or below its glass transition temperature from the substrate used. The firmer the adhesion of the cholesteric film to the substrate, the larger the mechanical stress to which the substrate must be subjected when the cholesteric layer is stripped. This results in high wear of the substrate material and consequently in increased production costs for the cholesteric films. Problems occur in particularly drastic form if layers having a thickness of less than 10 xcexcm, consisting of a non-brittle material, have to be stripped.
It is therefore an object of the invention to provide a method which enables platelets having a thickness of 0.2-1000 xcexcm and a diameter of 0.5-10,000 xcexcm to be produced without the abovementioned problems even from materials having high elongation at break and low ultimate tensile strength.
This object is achieved by a method wherein a material to be processed into platelets is applied in a flowable or gaseous state to a substrate, is consolidated, and removed from the substrate and subjected to a crushing or grinding process, wherein the material is applied to the substrate in succeeding layers, such that after its application, a lamina pack is present, which in its cross section perpendicular to its surface, has a plurality of individual laminae of a thickness of between 0.2-1000 xcexcm, and the lamina pack, having been stripped from the substrate, is treated in such a way that the material disintegrates into individual laminae and the individual laminae are reduced in size to a particle size of 0.5-10,000 xcexcm.